Recently, a PCM (pulse code modulation) technique has been developed and applied to a recording or reproducing system for both video discs and for audio discs. A recording of a PCM signal onto or off a disc is done optically or electrostatically.
In reproducing the PCM signal from a disc, optical pickups or electrostatic pickups are used to trace the recorded track of the disc while it is rotated at a very high speed.
The rotation of the disc must follow precisely the rotation of the turntable since the rotation speed of the turntable varies in accordance with a rotation speed control signal extracted from the reproduced PCM signal.
Therefore, it is desirable for the disc rotating apparatus not only to support the disc firmly on the turntable during the playing of a disc but also to allow the easy removal of the disc from the turntable after playing the disc.
Some conventional disc rotating apparatuses have been used. One of them has a clamp biased against the turntable by a spring. The clamp presses the disc against the turntable during playing. The spring, however, causes friction between the clamp and the spring so that a strong driving power for rotating the disc is needed. Wow and flutter characteristics caused by the rotation of the turntable can become worse due to the friction between the clamp and spring adversely effecting the rotation speed control.
Other devices use a permanent magnet as a clamp. The attractive force between the permanent magnet and the turntable made of a magnetic material holds the disc between them. However, it is difficult to remove the disc from the turntable because the permanent magnet is attracted to the turntable. Furthermore, the turntable must be fabricated of a magnetic material.